Basically, what has happened is my macbook is failing to turn on. It turns on and its just about to get to the login area were I can enter my password to get to the desktop, but then it shows a grey screen and the little preloader image and the preloader doesn't move it just freezes?

I've tried to turn it on so many times now and its failing to turn on. Anyone any idea of how to fix this or if this has happened to you before?